The present invention relates to a load drive which can be loaded into various vehicles including automobiles and more particularly to a load drive for driving various lamps in a vehicle loaded with a high-tension power supply unit under development in recent years.
There is a generally known automobile (i.e., so-called 14V vehicle) loaded with a power supply unit whose rated DC output ranges from 12V to 14V and which has an alternator of 14 volts (hereinafter called the ‘volt’ or ‘V’) and a 12V battery capable of charge and discharge. In the 14V vehicle, the load drive that is supplied with electric power from the power supply unit so as to drive various lamps such as a headlamp, a room lamp and the like generally operates to light each of the lamps by directly supplying an electric power of 12V-14V.
Recently, the development of a high-tension economical automobile (i.e., so-called 42V vehicle) is proceeding, the high-tension automobile being loaded with a power supply unit whose rated DC output ranges from 36V to 42V with a 42V motor/generator and a 36V battery capable of charge and discharge. With respect to the 42V vehicle, in a load drive under review for driving various lamps for use in the 14V vehicle, the collector terminal and the emitter terminal of an NPN switching power transistor, for example, are connected in series to a power supply line of DC 36V-42V connected to each lamp, and a rectangular-wave pulse voltage, for example, is applied to the base terminal of the switching power transistor so as to keep the electric power supplied from the emitter terminal of the switching power transistor to the lamp under PWM (i.e., Pulse Width Modulation) control.
However, for the various lamps, there are lamps directly replaceable by a vehicle user such as those for illuminating inside the vehicle including a room lamp, a map lamp, a foot lamp and so forth.
In the 42V vehicle provided with the load drive, tests were made for replacing the lamp for illuminating inside the vehicle while a power supply switch of the lamp therefor was held ON. Then confirmation was held up of a lamp damage phenomenon in which the filaments of new lamps were fused immediately after the lamps were installed due to pulse voltage including DC current component from 36V to 42V at the emitter terminal of the switching power transistor and as well as large current caused by the pulse voltage. The lamp damage phenomenon should be understood to mean that glass bulbs of lamps for illuminating inside vehicles may be broken because of filament fusion and that the load drive poses a serious problem to be solved.